1. Field of the Invention
The present invention relates to methods for manufacturing a solar cell module, and more particularly, relates to a method for manufacturing a solar cell module which comprises at least one photovoltaic element in which a transparent conductive layer forms an outermost surface part thereof at a light incident side, and a sealing resin layer formed on the transparent conductive layer.
2. Description of the Related Art
Various types of solar cells (photovoltaic elements), such as crystal silicon solar cells, polycrystal silicon solar cells, amorphous silicon solar cells, copper indium selenide solar cells, and compound semiconductor solar cells, have already been used. Among those mentioned above, since thin-film crystal silicon solar cells, compound semiconductor solar cells, and amorphous silicon solar cells can be manufactured at a relatively low cost and may be designed to have a larger surface area, research and development therefor have been aggressively performed.
In addition, among the solar cells mentioned above, since a thin film solar cell represented by an amorphous silicon solar cell, which is composed of a conductive metal substrate, silicon deposited thereon, and a transparent conductive layer made of a transparent conductive metal oxide thin film formed on the silicon, is lightweight and has superior impact resistance and flexibility, the thin film solar cell has been regarded as one of the prospective candidates used for forming a future solar cell module. However, in a manner different from that in the case in which silicon is deposited on a glass substrate, a transparent sealing material must be provided to cover a surface of a photovoltaic element at a light incident side for protection thereof. Accordingly, as a surface sealing material in contact with a photovoltaic element, for example, various thermoplastic and thermosetting transparent organic polymers, fluorinated resins, or hard coating materials capable of forming an inorganic film have been proposed. Some of the reasons for using the materials mentioned above are as follows. First, thermoplastic and thermosetting resins are inexpensive and can be formed into a sealing material having a large thickness sufficient for protecting a photovoltaic element at a low cost; second, since fluorinated resins have superior weather resistance, degradation caused by outdoor exposure for a long period of time is very small, and hence degradation in performance of a photovoltaic element can be minimized; and third, since hard coating materials are formed into an inorganic film, a very hard coating film having superior weather resistance and humidity resistance can be realized.
In addition, on a photovoltaic element, collector electrodes are formed from a conductive paste composed of a binder polymer and a conductive fine powder dispersed therein, so that current can be efficiently collected.
In a general solar cell module, a photovoltaic element group is formed from a plurality of photovoltaic elements, and collector electrodes are provided on each photovoltaic element. In addition, by a thermoplastic transparent organic resin, the photovoltaic element group is sealed, and in addition, a thin fluorinated polymer layer and an insulating layer are bonded to each other. In this example, the same organic resin as that provided at a light incident surface side is also used at a rear surface side. More particularly, the thin fluorinated polymer layer is made of a fluorinated resin film such as an ethylene-tetrafluoroethylene copolymer (ETFE) film or a poly(vinyl fluoride) film (PVF); the thermoplastic transparent organic resin is an ethylene-vinyl acetate copolymer (EVA), a butyral resin, or the like; the collector electrode is formed from a conductive paste using a conventionally known resin as a binder polymer; and for the insulating layer, various organic resin films, such as a nylon film and an aluminum laminated teddler film, may be used. In this example, the thermoplastic transparent organic resin is used as an adhesive between the photovoltaic element, the fluorinated resin film, and the insulating layer and is also used as a filling material for protecting the solar cell module from damage such as scratching and/or impact done thereto from the outside.
However, in the structure of the conventional surface coating material, an interface formed of an inorganic material and an organic material is present between the photovoltaic element made of an inorganic layer and the transparent organic polymer resin, which forms an organic layer, and hence, due to the difference in properties therebetween, such as a coefficient of thermal expansion, a glass transition temperature, hydrophilic and hydrophobic properties, and water absorption, the adhesion between the layers has not been satisfactory.
Accordingly, when photovoltaic elements covered with a coating layer are used outside for a long period of time as a solar cell module, due to the insufficient adhesion between the photovoltaic elements and the transparent organic polymer resin and a thermal stress caused by severe changes in temperature and humidity, the photovoltaic element and the transparent organic polymer resin will be peeled away from each other, thereby causing a serious problem in view of the quality of the solar cell module.
In addition, even when a hard coating material capable of forming an inorganic coating film is applied, a coating film composed of only inorganic materials cannot be formed, and hence the adhesion to a photovoltaic element is still insufficient. Accordingly, when a hard coating film having a high hardness is formed from a hard coating material, by even a small difference in coefficient of thermal expansion between the hard coating film and a photovoltaic element, cracking and/or peeling may be liable to occur under high temperature conditions in some cases.
In order to solve the problems described above, a so-called integral blend method has been performed in which various coupling agents made, for example, of silane compounds and/or organic titanate compounds are added to a surface sealing material which is in direct contact with a photovoltaic element. However, by the method described above, for example, the following problems have occurred. That is, (1) since the content of a coupling agent becomes large such as one percent by weight or more, the properties of a surface sealing material resin are changed, and hence the durability thereof may be decreased in some cases; (2) when contained in a paint for coating, a coupling agent will compete with a leveling agent added thereto beforehand, and as a result, it may not be sufficiently supplied to the interface in some cases; (3) when added to a thermoplastic or a thermosetting resin in advance, a coupling agent is decomposed in heating for hot molding or heat curing; and (4) when a sealing material resin containing a coupling agent is held for a long period of time, gelation thereof advances as a reaction between the coupling agent and the resin gradually proceeds, and hence coating must be performed as quick as possible after the coupling agent is added to the sealing material resin.
In addition, Japanese Unexamined Patent Application Publication No. 7-38126 has disclosed that the adhesion between a sealing material and a surface of a photovoltaic element is improved by forming a monomolecular layer made of a silane coupling agent thereon. However, since the bonding between the photovoltaic element and the silane coupling agent is gradually hydrolyzed with moisture present in the air, the effect described above is limited in outdoor exposure performed for a long period of time.
On the other hand, in order to efficiently collect current generated in the semiconductor active layer, collector electrodes which collectively form a comb shape are provided in many cases on a photovoltaic element by using a conductive paste containing an organic polymer as a binder polymer. However, by the conventional method described above for forming collector electrodes, due to insufficient adhesion between the photovoltaic element and the binder polymer, difference in coefficient of thermal expansion therebetween, and the like, peeling is liable to occur between the photovoltaic element and the collector electrode. In addition, this occurrence of peeling increases contact resistance between the photovoltaic element and the collector electrode and is partly responsible for degradation in performance of the photovoltaic element. This problem described above is particularly serious when the photovoltaic elements are used outside for a long period of time as a solar cell module, and the contact resistance is liable to be increased by the change in temperature; hence, it has been difficult to guarantee the performance, such as conversion efficiency, of a solar cell module for a long period of time.
Accordingly, in order to improve the adhesion between a conductive paste and a photovoltaic element and to suppress the increase in contact resistance between a photovoltaic element and collector electrodes, as is the case with the use of the surface sealing material, addition of coupling agents to a conductive paste has been performed. For example, in order to improve the stability of an ohmic contact with time, Japanese Unexamined Patent Application Publication No. 2-170473 proposes to use a silane coupling agent at an interface between a thick electrode made of a conductive paste and a silicon film at a light transmitting surface side, the silicon film being a constituent element of an amorphous silicon solar cell formed on a light transparent substrate.
However, the use of coupling agents has several problems as is the case with the surface sealing material described above.